There are 3 ways of attaching Ai to weapons and vehicles.
If your Ai is only ever going to use a certian weapon or vehicle then I use the item attributes to attach him.
Ai will always go to the highest value position, ie, driver then gunner then passenger
From that screenshot you will see goto player and goto group, neither of these work in a multiplayer coop map.
You can use an event
Scroll down the 1st box to the ssn number of your Ai
scroll down the second box as above.
Then scroll down the 3rd box to the ssn number of the weapon or vehicle he should use.
You can use a WAC scriot:
if never then
ssn2ssn(Ai ssn number , weapon ssn number)
ssnuse( Ai ssn number)
endif
This has the added benefit that the Ai can be anywhere in the map and he is teleported directly to the weapon/vehicle.